Method for producing aluminumsilicon alloys



Patented Mar. 15, 1949 Albert Reg-net, ,Pr-aha -Smichov, Czechoslovakia,assignor to Spole'k Pro Chemickou a ,Hutni Vyrobu,

also named UnitedJChemical and Metallurgical Works, Ltd Pi-aha,Czechoslovakia, acorporation' of Czechoslovakia I No Drawing.vApplication -.I -une 26, 1946, Serial No. 679,592. Inv.GzechoslovakiaJuly 8, 1940 Section 1, Public liaw 690,-August 8,1946.Patent expires July 8, 1960 aclaims. 1

My invention relates to a methodforiproducing industrially importantaluminum si'licon alloys.

It hasbeen proposed to prepare a eutectic or: a supereutecticaluminum-silicon alloy by fractional crystallization of acrude alloycontaining more than 12% Si and accompanying'metallic impurities. Ofthese impurities, iron,'titanium and carbides have 'anespeciallydeleterious :eiiect on the end products preparedfromsuchalloys. However,inasmuch as the two lastimentioned impurities can be adequatelyeliminated by such fractional crystallization, there is no further needto consider them hereinafter.

The crude alloy is prepared from oxides ofaluminum and silicon,forinstance either by electrothermic reduction of -a mixture of kaolinand aluminum oxide with the aid of carbon, or by electrolysis of a fusedbath ofbauxite, to which may be added some earth. As is well known, theseparation'may beachieved in several different ways. According to onemethod, the crude alloy is subjected to fractionalcrystallization sothat it is allowed to cool gradually from =a-completely iuse'd state toa certaintemperature which'lies in the liquidus-solidus reg-ion andwhich temperature corresponds "to the desired composition of theliquidus, whereupon the resulting crystals rich in siliconare'separated, either by centrifuging-or by vacuum or pressurefiltration,-'fromthe' mother melt-which is concomitantly enriched inaluminum,

According to another method, theseparation is achieved by filtration,whereby thecrude alloy, previously treated thermally'soas to yield aslarge silicon crystals as possible, is carefuily-*heated abovetheeutectic temperature and the sieved out liquid part is separated fromthe solid phase, by filter-pressing for example.

The composition of theliquidus separatedin such operations is controlledby the equilibrium diagram of the system concerned, according to whichAl-SiFe alloys form a ternary eutectic which melts at 577 C. and whichcontains,11.6% Si, and 0.80% Fe, the balance being Al. However, asaccordin to this diagram the technical crude alloys contain notonlysilicon, but-also iron inasupereutectic amount, it is apparent thatthe liquidus-separated from thecrude alloy at a-tenlperature slightlyabove the eutectical will contain the smallest amounts of these metals,whereas melts isolated at highertemperatures ---wil1 .contain not onlymore silicon, but also more 'iron. Experiments performed with thisobject in mind completely confirmed the correctness of the equilibriumdiagram. Thus, a liquidus separated from the-crude alloy at 579 C.showed practically an identical composition with that of the above saideutectic, whereasa separation performed at 588 C. -yielded a productcontaining 13.6% Si and 0.90% Fe.

Although according to some patents suchproducts, isolated from crude andimpure alloys at low temperatures,-were considered to be equivalent tothe .soecalled silumin, a castingalloy of. considerable technicalimportance and of'precisely formulated. composition, nevertheless infact they merely complied with the specification regarding the. requiredcontent of silicon, but did not satisfy the purity requirement becausethey contained about -=0.85% Fe, representing an iron contentalmostztwice as high as the admissible maximum. Experience teaches thatan iron content in excess ofi 0.5% i has a considerable harmful effecton the mechanical properties of silumin-type alloys. This; circumstance,1 therefore, i was the principal cause why the above describedproduction method never-became popular inindustrial practice.

"11511136 now'been discovered that the iron content is re'duced'andmechanical properties are improved if a certain amount of chromium,cobalt,

or manganese is alloyed with the crude alloy beforehand, and onlythisadjusted alloy is then subjected to the separation process at atemperaturejust above the eutectical. However, it is equally:advantageous to'alloy'the Cr, 00 or Mn only into theliquidus which wasfirst separated from .CI'LKXG'BIHOYS by known methods, without theaddition of heavy metals, at a temperature close to the eutectical. Thisliquidus thus contained thetprescribed-amountof silicon but also had aninadmissi'bly high iron content. By the following separation of thisintermediate product, at substantially the same temperature as that ofthe firstoperatiomthe iron content of they product is reduced to thedesired extent without further changing the ratio of aluminum tosilicon.

The said metals are'alloyed either by themselves .orin a mixture, andthe added material advantageously amounts to 0.5 to 2 times theamount-of iron present in the original alloy. Under such conditions theliquidus isolated at a temperature close to the eutectic temperaturecontains besides 12-13.5% Si only 0.25-0.40%--Fe and .0;3-0.5% of? thosemetals which were added tolthezoriginal mixture in order to reduce theiron content. The, presence ofthese small amountsof heavy metals isin noway harmful to the quality of the product, on the contrary, as is evenilluscomposition of silumin, such heavy metals have a very favorableeffect in that they improve the metals can be added immediately to thecrude alloys so that the iron is eliminated in the desired amountsimultaneously with silicon, or they can be added only to the alloy ofthe finally desired silicon content but which still contains too muchiron. This excessive iron is then eliminated in a separate process.Furthermore, the new process can be employed also for the regenerationof aluminum-silicon alloy scrap if such scrap is contaminated with aninadmissibly high content of iron which is dissolved in the alloy.Inasmuch as it is then usually desired to eliminate the iron only, butnot to change the aluminumsilicon ratio, in such instances theseparation is conducted at a temperature just sufiicient to keep bothprincipal metals of the alloy in a molten state.

It is possible to introduce the heavy metals into the original crude andimpure alloys by direct alloying of the pure metals or of appropriateprealloys, or by an exchange reaction between chlorides or otherconvenient compounds of the heavy metals and a melt of the originalalloy, or finally by the simultaneous reduction of the oxides of theheavy metals in the case of electrothermic or electrolytic processes.

My new method for reducing the iron content of aluminum-silicon alloysis entirely distinct from another known method which, by the addition ofMn, Co or Cr, merely reduces the harmful effect that an excessive ironcontent in aluminum-silicon alloys has on castings. Alloys modified bythe 01d method are suited only for pressure injected die castings, but,in view of their high Fe content they are not usable for castings inmoulds or in sand, because the iron has not been eliminated but merelytransformed into a structurally less harmful form. Besides, in the oldprocess the improving effect of thesaid heavy metals can be utilizedonly up to a certain maximum iron content and therefore the alloys ofparticularly high iron content were unusable even for die-castings.

The following examples are recited here in order to illustrate thepresent invention:

Examples 1. A crude aluminum alloy, which was pre pared in an arcfurnace and from which carbides were eliminated previously, and whichcontained 30.5% silicon and 1.3% iron, was gradually cooled after fusionso that the separating crystals, consisting predominantly of silicon,formed as large individual crystals as possible. As soon as thetemperature dropped to 588 C., that is 11 C. above the eutectictemperature of the Al-Si-Fe system, the alloy was subjected to 4 apressure filtration. The isolated liquid phase had a composition of 0.9%Fe, 13.6% Si, and the balance Al.

2. The crude alloy of the above said composition was subjected to afurther separation process in which, however, the temperature beforefiltration was allowed to drop as far as 579 C., i. e. 2 0. above theeutectic temperature. The isolated liquid phase contained 0.75% Fe,11.6% Si, balance A].

3. To this same alloy manganese was added in an amount corresponding to1.3 times the amount of iron contained therein. By filtration at atemperature of 588 C. a fraction was obtained containing 0.48% Fe, 0.38%Mn, 13.5% Si, balance Al. 5

4. The process illustrated by Example 3 was repeated with the differencethat the melt was cooled to 579 C. before filtration. The resultingliquidus contained then 0.36% Fe, 0.28% Mn. 11.6% Si, balance Al.

Although I have described certain specific em-- bodiments of my newmethod, I am fully aware that many variations and modifications may bemade in the details of procedure above set forth without departing fromthe spirit of my invention. My invention, therefore, is not to berestricted except insofar as is necessitated by the prior art and by thespirit of the appended claims.

What I claim is:

1. A process for decreasing the impurity of iron in aluminum-siliconalloys to less than 0.5% said process comprising melting the impurealuminum-silicon alloy, adding thereto at least one metal selected fromthe group consisting of Mn, Co, and Cr,'in amounts equal to about 0.5 to2 times the total weight of iron present in the impure alloy, andthereafter slowly cooling said alloy and separating the soliduscontaining the major portion of iron from the liquid phase containingless than 0.5% iron at a temperature above the fusion point of theeutectic.

2. A process for decreasing the impurity of iron in aluminum-siliconalloys to less than 0.5% said process comprising melting the impurealuminum-silicon alloy, adding thereto at least one metal selectedfromthe group consisting of Mn, Co, and Cr, in amounts equal to about 0.5 to2 times the total weight of iron present in the impure alloy, andthereafter subjecting said alloy to fractional crystallization andseparating the solidus containing the major portion of iron from theliquid phase containing less than 0.5% iron at a temperature above thefusion point of the eutectic.

ALBERT REGNER.

REFERENCES CITED The following references are of record in the file of,this patent:

UNITED STATES PATENTS OTHER REFERENCES Metal Industry (London), vol. 52(1938), pages 631-636.

